It varies from dog to dog but in GDSs normally I consider my females fully "Mature" around 2 to 2 1/2 years of age - even though size wise they look mature a lot sooner.
Typically males mature even slower - I consider my "boys" to be "men" when they're around 3 years of age.
At these ages both males and females have had time to grow to their adult height but they have also filled out in their frames to a more mature adult size and weight - plus they have matured mentally and are no longer an adult sized dog with a puppy aged brain.